1. Field of the Invention
This invention relates to a method for improving the light-fastness (color fastness to light) of reversible thermochromic compositions at the time of color development by bringing into presence an electron accepting, special light-fastness providing agent.
2. Description of the Related Art
Thermochromic compositions comprised of a combination of electron donating compounds with electron accepting compounds are known in the art as disclosed, e.g., in U.S. Pat. No. 3,560,229. In such compositions, their metachromatic temperatures depend on the types of the both compounds used in combination, and hence, it has been very difficult to obtain compositions capable of varying in color at the desired temperature.
The present inventors have invented a method in which a specific alcohol, ester or ketone is used as a metachromatic temperature adjuster to adjust the metachromatic temperature to the desired temperature (Japanese Patent Publications No. 51-44706, No. 51-44708, No. 52-7764 and No. 6-59746).
The invention previously made by the present inventors is an epock-making invention in view of the advantages such that the metachromatic temperature can be controlled to any desired temperature even if the combination of an electron donating compound with an electron accepting compound is not varied.
However, the invention made by the combination of these electron donating compound and electron accepting compound has a disadvantage that the electron donating compound does not have a high light-fastness and hence any thermochromic compositions making use of it can not have a high light-fastness. Accordingly, the color having varied may become faded by light or, in an extreme instance, may become completely decolored.
In conventional attempts, ultraviolet light absorbers are mixed so that light-fastness can be imparted to compounds. When, however, an ultraviolet light absorber is mixed in thermochromic compositions comprised of an electron donating compound and an electron accepting compound, it brings about an improvement in light-fastness when the compositions are in the color-extinguished state but is almost not effective when they are in the color-developed state, and is not effective at all especially with regard to indirect light. When a metallic luster pigment and an ultraviolet light absorber are used in combination to intercept part of visible light rays, they bring about a dramatic improvement in light-fastness when the compositions are in the color-extinguished state but are less effective in the color-developed state, and are not effective at all especially with regard to indirect light. Also, hindered amine type, hindered phenol type, sulfur type or nickel complex type light stabilizers are almost not effective either when the compositions are in the color-extinguished state or when they are in the color-developed state.